There are some instructions on Garmin's website here.
This is what it says....
Importing GPX Files with the Garmin Drive App
You can import data, such as waypoints and routes, in GPX file format using the Garmin Drive™ app. For example, you can send waypoints or routes received in an email to your RV 890 / Camper 890 device.
On your smartphone, select a .gpx file and open it with the Garmin Drive app.
Follow the on-screen instructions in the Garmin Drive app.
On your RV 890 / Camper 890 device, select an option:
To view imported waypoints, select Where To? > Saved.
To view imported routes, select Apps icon > Navigation > Trip Planner > Saved Trips.
I think that although I'm all mapped out on RV TripWizard, all I am going to do is input the campgrounds I plan to travel to, probably a day or 2 before the travel day, and optimize the route on the RV 890 using their routing and then use "Shape Route" to what I like, RV TW says and what Garmin recommends. Sees easier than the whole export GPX, import, etc....